To control the steam flow to a steam turbine, for instance, or steam reduction conducts, a control valve which opens and throttles the steam flow is required for controlling the rotation speed of the steam turbine, and a fast-closing valve which, during control errors or some other, outer fault, can be moved from a totally open to a totally closed position within a very short space of time, in the order of 0.2 s, to completely stop the steam flow is required. The fast-closing valve and the control valve are controlled independently of each other, that is the fast-closing valve must be able to close independently of the position of the control valve.
According to the prior art, these valves can be arranged in respective valve housing in a steam flow channel, the fast-closing valve being arranged upstream of the control valve. The valve housing of the fast-closing valve as well as the valve housing of the control valve form a perpendicular bend of the steam flow channel, that is the flow is redirected twice by 90.degree.. A steam sieve is concentrically arranged around the fast-closing valve. The steam sieve has as its task to sieve the steam and to separate particles that could damage the turbine. This construction has the following drawbacks. Firstly, the structure becomes costly as it requires two valve housings. These two valve housings are also space-requiring and, accordingly, occupy a lot of space around the turbine. Furthermore, the double redirection of the steam flow leads to an important drop of pressure flow losses and a high noise level.
SE-B-411 636 shows a combined fast-closing and control valve for steam turbine arrangements. The fast-closing and control valve bodies are arranged in a common housing and, by means of a respective actuator, are controllable independently of each other through respective spindles. Furthermore, they are arranged to be displaceable towards coaxial, adjacent sealing surfaces of a common valve seat. The fast-closing valve body is designed as a bell into the inner room of which the control valve body projects coaxially. Also according to this solution the steam flow must be redirected twice by 90.degree.. Thereby, the fast-closing valve body projects into the flow path and is brought downwards from above, towards the valve seat during closure. The control valve body is rigidly connected to a spindle which extends up and through the valve seat, that is it runs in the flow path. Also in this solution important flow losses are obtained as the flow is twice redirected. Furthermore, the flow losses become important due to the fact that the control valve body extends upwards through the valve seat. To compensate for the area occupied by the spindle of the control valve body, the passage opening of the valve seat must be enlarged. Furthermore, the structure requires large amount of space as the actuators of the valves are located at a respective side of the valve housing.
DE-A-2 533 519 shows another type of valve for steam turbines, with an inlet chamber and an outlet chamber for the steam flow and a common valve housing comprising a fast-closing valve, an outblow valve connected thereto and a control valve, the valve bodies of which are coaxially arranged. This valve presents an actuator with a spindle belonging thereto for the fast-closing valve and the outblow valve. As the fast-closing valve is opened the outblow valve is closed and vice versa. Furthermore, the valve body of the fast-closing valve is arranged to be inserted in an inner spacing of the control valve designed as a bell. This structure also redirects the steam flow twice by 90.degree. and presents a very unfavorable design for the steam flow. Furthermore, the control spindle of a valve body extends through the valve seat and, thus, through the steam flow. Therefore, also in this structure, important flow losses and an important fall in pressure are obtained. Furthermore, the structure is space-demanding due to the two actuators and the complicated design of the housing.
DE-A-2 523 297 shows a combined fast-closing and control valve for steam turbines, the valve bodies of which are coaxially arranged in a common valve housing. The valve bodies act from respective sides of a common valve seat and are controlled independently of each other by means of respective actuators with a spindle belonging thereto for the control valve, and a further actuator through a respective spindle. Also in this case the flow is redirected twice by 90.degree., so that the flow losses become important, and the actuators of the valves are arranged on respective sides of the valve housing, which requires space.